This invention relates to a multi-layer printed circuit board with high dimensional precision and a process for the production thereof.
An increasing tendency is seen recently to greater compactness and densification of the multi-layer printed circuit boards used in the industrial and business machines such as computers. In line with this, there are required higher heat resistance of the boards to cope with increase of heat release from elements, e.g., LSI, in use, finer pattern to accommodate increased wiring density, higher dimensional stability between layers in response to the increased number of layers and higher through-hole reliability. As a material that can answer to such requirements, polyimide type materials have come to be used in place of the conventional epoxy type materials which are rather poor in heat resistance. Dominant among such polyimide materials are aminobismaleimide resins and their epoxy-modified resins. In production of a printed circuit board, such an epoxy or polyimide resin is impregnated in reinforcing fabrics to form prepregs and separately made unit circuit sheets are laminated via such prepreg resin layers. The heating temperature used in the heat and pressure molding for lamination is usually of the order of 170.degree.-180.degree. C., and the resultantly obtained cured product has a glass transition temperature (Tg) of the order of 120.degree.-160.degree. C. which is lower than said molding temperature. Glass transition temperature (Tg) is used as an index of heat resistance, but Tg of the order of 120.degree.-160.degree. C. mentioned above cannot provide satisfactory heat resistance. In order to obtained sufficient heat resistance, it needs to perform additional 2- to 3-hour post-curing by heating the molding to 200.degree.-240.degree. C. to elevate Tg to a level above 170.degree. C., preferably to a level of around 200.degree. C. However, post-curing under such elevated temperatures tends to cause dislocation among the layered circuit boards and the reduction of adhesive strength between the inner copper foil and prepreg, resulting in an adverse effect to the dimensional precision of the product. While removal of such evil effects of post-curing has been desired, no positive solution to this problem has been available to date.
In the hope of solving such problem, the present inventors have pursued studies on a relationship between the heating temperature (T.sub.L) in the working process, including the heat and pressure molding for lamination of circuit boards and post-curing, and the glass transition temperature (Tg) of the cured resin in the boards, and found that said problem can be solved by a consideration from such a new standpoint. The present invention was reached as a result of such studies.